This invention, in the production of a porous cross-linked polymer forming continuous pores in the surface and the interior thereof and consequently possessing continued cells (hereinafter occasionally referred to as xe2x80x9copen cellsxe2x80x9d) by polymerizing a water in oil type high internal phase emulsion (hereinafter occasionally referred to briefly as xe2x80x9cHIPExe2x80x9d), relates to a method for producing the porous cross-linked polymer by curing the HIPE in a very brief time. More particularly, this invention relates to a method for the production of a porous cross-linked polymer containing open cells and finding extensive utility in such applications as (1) liquid absorbent materials such as, for example, {circle around (1)} core materials in disposable diapers to be used for absorbing body fluid such as urine and blood and {circle around (2)} agents for waste water and waste oil disposal and agents for water solvent disposal to be used for absorbing water, aqueous liquid, oil, and organic solvents, (2) energy absorbent materials such as, for example, sound insulating materials and heat insulators in automobiles and buildings to be used for absorbing sound and heat, and (3) chemical impregnating substrates such as, for example, household products impregnated with aromatic agents, detergents, lustering agents, surface protecting agents, and flame-retarding agents.
The term xe2x80x9cHIPExe2x80x9d refers to an emulsion which has a water phase as a dispersion phase (internal phase) and an oil phase as an external phase at a ratio (W/O ratio) of not less than about 3/1. The fact that a porous cross-linked polymer is produced by polymerizing this HIPE has been known to the art.
While the porous cross-linked polymer which is produced by using a foaming agent without preparing the HIPE readily constitutes itself a foam of discrete cells of a comparatively large diameter, the method for producing the porous cross-linked polymer from the HIPE (hereinafter occasionally referred to simply as xe2x80x9cHIPE methodxe2x80x9d) excels as a process for the manufacture of a low-density foam of open cells of a minute diameter.
Methods for producing a porous cross-linked polymer from the HIPE are disclosed, for example, in JP-A-57-198,713, JP-A-62-250,002, U.S. Pat. Nos. 5,252,619, and 5,189,070.
In JP-A-57-198,713 and JP-A-62-250,002, a method which comprises preparing a HIPE containing a water-soluble and/or oil-soluble polymerization initiator and thermally polymerizing this HIPE at 50xc2x0 C. or 60xc2x0 C. for a period in the range of 8 hours to 72 hours has been disclosed. Then, U.S. Pat. No. 5,210,104 discloses a method which comprises adding a polymerization initiator after preparing a HIPE, U.S. Pat. No. 5,252,619 a method which comprises preparing a HIPE containing a polymerization initiator and then polymerizing the HIPE at a temperature of not less than 90xc2x0 C., and U.S. Pat. No. 5,089,070 a method which comprises forming a gel possessing a prescribed modulus of elasticity in shear from an emulsion at a temperature in the range of 20-65xc2x0 C. and thereafter polymerizing the gel at a temperature of not less than 70xc2x0 C.
The methods which are disclosed in JP-A-57-198,713 and JP-A-62-250,002, however, require a long time for the polymerization and suffers from poor efficiency of production. When the polymerization is performed at high temperatures as disclosed in U.S. Pat. Nos. 5,252,619 and 5,189,070, the polymerization requires a period totaling several hours and, depending on the conditions, tends to impair the stability of the HIPE and induce liberation of a large amount of water, and possibly fails to obtain a porous cross-linked polymer possessing a stated pore diameter. U.S. Pat. No. 5,210,104 has a mention to the effect that the polymerization requires several hours notwithstanding the HIPE is emulsified with improved stability owing to the addition of a polymerization initiator after the preparation of the HIPE.
It is, therefore, an object of this invention to solve the technical problems enumerated above and to provide a method for the production of a porous cross-linked polymer which enables the HIPE to be polymerized (cured) in an exceptionally brief time of not more than 30 minutes, preferably not more than 10 minutes, for example, without impairing the stability of the HIPE.
The present inventors, after pursuing a diligent study in search of a method for the production of a porous cross-linked polymer in a very brief time by the HIPE method, have discovered that when a HIPE is formed in a comparatively small thickness falling in a specific range and the HIPE of this thickness is heated at a specific temperature-increasing rate until reaching a target curing temperature and then cured, the liberation of water observed during the cure at the conventional increased temperature is not observed at all and a porous cross-linked polymer of an uniform thickness can be obtained and this porous cross-linked polymer excels in absorption properties. This invention has been perfected as a result.
To be specific, the object mentioned above is accomplished by the following items (1)-(13).
(1) In the production of a porous cross-linked polymer by the steps of mixing an oil phase containing a polymerizing monomer, a cross-linking monomer, and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion and heating this emulsion till a prescribed curing temperature thereby polymerizing the emulsion, a method for the production of the porous cross-linked polymer characterized by forming the emulsion in a thickness of not more than 50 mm and heating the emulsion at a temperature-increasing rate of not less than 5xc2x0 C./minute.
(2) In the production of a porous cross-linked polymer by the steps of mixing an oil phase containing a polymerizing monomer, a cross-linking monomer, and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion and heating this emulsion till a prescribed curing temperature thereby polymerizing the emulsion, a method for the production of the porous cross-linked polymer characterized by forming the emulsion in the shape of a film or a sheet and heating the emulsion at a temperature-increasing rate of not less than 5xc2x0 C./minute.
(3) A method set forth in (1) or (2) above, wherein the temperature-increasing rate in the heating of the emulsion is in the range of 5-60xc2x0 C./minute.
(4) A method set forth in any of (1)-(3) above, wherein the difference between the forming temperature of the emulsion and the curing temperature of the emulsion is in the range of 2-50xc2x0 C.
(5) A method set forth in any of (1)-(4) above, wherein the forming temperature of the emulsion is in the range of 40-95xc2x0 C.
(6) A method set forth in any of (1)-(5) above, wherein the curing temperature of the emulsion is in the range of 80-110xc2x0 C.
(7) A method set forth in any of (1)-(6) above, wherein the time for polymerizing the emulsion is within 30 minutes.
(8) In the production of a porous cross-linked polymer by the steps of mixing an oil phase containing a monomer component comprising a polymerizing monomer and a cross-linking monomer and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion and polymerizing this emulsion in the presence of a polymerization initiator, a method for the production of the porous cross-linked polymer characterized by forming the emulsion in a thickness of not more than 50 mm or in the shape of a film or a sheet, keeping the time of polymerization within 30 minutes, and controlling the amount of the polymerization initiator which completes decomposition within the polymerization time in the range of 0.05-2.0 mol % based on the amount of the monomer component.
(9) In the production of a porous cross-linked polymer by the steps of mixing an oil phase containing a monomer component comprising a polymerizing monomer and a cross-linking monomer and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion and polymerizing this emulsion in the presence of a polymerization initiator, a method for the production of the porous cross-linked polymer characterized by subjecting the emulsion to a series of operation continuing from the step of forming through the step of polymerizing, forming the emulsion in a thickness of not more than 50 mm or in the shape of a film or a sheet, keeping the time of polymerization within 30 minutes, and controlling the amount of the polymerization initiator which completes decomposition within the polymerization time in the range of 0.05-2.0 mol % based on the amount of the monomer component.
(10) A method for continuous production of a porous cross-linked polymer, characterized by mixing an oil phase containing a monomer component comprising a polymerizing monomer and a cross-linking monomer and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion, continuously forming the emulsion, then heating the formed emulsion at a temperature-increasing rate of not less than 5xc2x0 C./minute till a prescribed curing temperature, and thereafter polymerizing the heated emulsion.
(11) In the production of a porous cross-linked polymer by the steps of mixing an oil phase containing a polymerizing monomer, a cross-linking monomer, and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion and heating this emulsion till a prescribed curing temperature thereby polymerizing the emulsion, a method for the production of the porous cross-linked polymer characterized by forming the emulsion in a thickness of not more than 50 mm or in the shape of a film or a sheet and the formed emulsion is heated for temperature increase by the use of an active thermal energy ray.
(12) A method set forth in (11) above, wherein the active thermal energy ray is a near infrared ray.
(13) A method for continuous production of a-porous cross-linked polymer, characterized by mixing an oil phase containing a monomer component comprising a polymerizing monomer and a cross-linking monomer and a surfactant as essential components and a water phase containing water as an essential component thereby obtaining a water in oil type high internal phase emulsion, continuously forming the emulsion, then heating the formed emulsion by the use of an active thermal energy ray, and thereafter polymerizing the emulsion at a curing temperature.